Mice can enter through a 1/4 " hole
most likely a gas pipe or along the sill wrote in message

*** the former exterminator speaks the truth, naturally. finding all
the holes is tough, unless you have enough of them to follow a steady
trail of droppings. in my house they use the pipes and dishwasher hose
under my kitchen sink as bridges, which means they largely avoid the
traps. i did learn in Philadelphia to use steel wool to plug any holes
you can find. tried it here and, amazingly, it's apparently a
universal that mice cannot tolerate chewing through steel wool (here i
thought they taught them that in the public schools back east).

again, that only works if you can find the holes.